1. Field of the Invention
The present invention relates to a process and system for extracting sulfur from a gas stream containing hydrogen sulfide. More particularly, the present invention relates to a process and system for desulfurizing ammonia acid gases containing hydrogen sulfide.
2. Description of Prior Art
Refinery streams are typically desulfurized by the Claus process wherein elemental sulfur is produced by reacting hydrogen sulfide and sulfur dioxide in the presence of a catalyst. The Claus system uses a combustion chamber which, at 950.degree. C. to 1,350.degree. C. (1742.degree.-2462.degree. F.), converts 50 to 70% of sulfur contained in the feed gas into elemental sulfur. Sulfur is condensed by cooling the reaction gas to a temperature below the dew point of sulfur after which the remaining gas is heated and further reacted over a catalyst. Normally, the gas passes through at least two such Claus catalyst stages.
The different stages of the process may be represented by the following equations: EQU H.sub.2 S+3/2O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3S+2H.sub.2 O
The overall reaction is: EQU 3H.sub.2 S+30.sub.2 .fwdarw.3S+3H.sub.2 O
Claus exhaust gas still contains small amounts of H.sub.2 S, SO.sub.2, CS.sub.2, carbon oxysulfide, CO, and elemental sulfur in the form of a vapor or mist. The exhaust gas generally is subjected to post-combustion to convert substantially everything to SO.sub.2 and then further purified by Claus after-treatments.
Ammonia acid gases typically are combusted substoichiometrically at about 2300.degree. F. at the front of the sulfur plant combustion chamber to completely destroy the ammonia. A portion of a clean acid gas (ammonia free) is also introduced along with the ammonia acid gas to control the temperature. The processing of the ammonia acid gas in the combustion chamber increases the sulfur plant size due to the increase in volume of the gases that need to be processed by the sulfur plant. For example, processing of ammonia may increase the hydraulic size of the plant by 20 to 50% based on the amount of ammonia acid gas that is processed. Further, ammonia that is not destroyed in the combustion chamber will form salts. Ammonia and sulfur dioxide react to form a very dense white smog of ammonium hydrosulfide. Ammonia in significant concentration of CO.sub.2 will form ammonium bicarbonates. These salts will lay down to plug sulfur seal legs, sulfur condensers, heat exchangers and reactor beds. This salt problem reduces the reliability of the sulfur plant.